Autism has been shown to be highly heritable, with heritability estimated at over 90 percent. Estimates for the number of genes underlying autism range from 3 to over 15 and there is evidence for genetic heterogeneity. Although there are several genome-wide linkage analysis studies that have been completed, there is a disappointing concordance in linked regions between the studies. Chromosomal regions where there is more agreement include 7q and 2q. The evidence for linkage to chromosome 2 was observed in our studies and in additional studies. The current application represents a focused attempt to identify an autism susceptibility gene on 2q. The specific aims are as follows: Specific Aim 1: Identify an additional 150 families with at least two family members, usually sibs, affected with autism. [This will be carried out with Core B/Clinical Core.] Specific Aim 2: Genotype samples across the 2q region and analyze the data for linkage to autism. Specific Aim 3: Genotype polymorphisms in genes of interest across the 2q region and carry out association studies.